Many CMOS (complementary metal oxide semiconductor) integrated circuits require small amounts of on-chip nonvolatile memory (NVM). Typical applications include storing security settings, RFID (radio frequency identification) data, system configurations, serial numbers, calibration and trim settings, and others. For reasons of cost and yield, the ideal NVM should be in state-of-the-art logic CMOS with zero additional process masks. Unfortunately, applications requiring relatively small amounts of NVM (a few hundred words) have been largely neglected by the major memory manufacturers as they focus on developing customized NVM processes that yield ever-increasing memory densities (e.g. 256 Mb Flash). Consequently, CMOS designers requiring small amounts of nonvolatile storage must (1) use technologies such as on-chip fuses, (2) pay the cost and absorb the yield degradation associated with using high-density embedded NVM, (3) resort to off-chip storage, or (4) use SRAM (static random access memory) storage powered by an associated battery backup.
Designers needing small amounts of NVM in highly integrated CMOS applications face some unpleasant tradeoffs. The obvious approach is to use a CMOS process with embedded NVM. Unfortunately, embedded NVM processes are burdened not only with higher wafer costs, but also tend to be older-generation technology. The higher cost is due to the fact that NVM processes generally require additional masks and fabrication steps (e.g., to obtain a second polysilicon layer). The older-generation technology arises because adding NVM to a logic process takes time and testing, so NVM processes typically lag the state-of-the-art by up to a year. The result can be that, for a precious few NVM bits, an entire CMOS chip will have higher cost and reduced performance.
One alternative to embedded NVM processes is to use fuses (or anti-fuses) that are either laser or electrically programmed. Applications requiring one-time programming may find this alternative attractive, but significant technology issues such as fuse “healing” and programming cost remain problematic. Furthermore, fuses and antifuses are often unavailable in state-of-the-art CMOS processes.
Another option is to use an off-chip solution such as a separate NVM chip or battery backup for on-chip SRAM. Unfortunately, this solution requires additional devices and, in the case of off-chip NVM, exposes the data to potential hacking. The benefit, of course, is that designers can implement the rest of the chip in a leading-edge technology without incurring the overhead of an NVM process. The disadvantage is higher cost, both in PCB (printed circuit board) area and parts count.
What CMOS designers need is an NVM capability in state-of-the-art logic CMOS.